Ligno-cellulosic feedstocks may be converted to useful compounds, such as biofuels and bio-chemicals, but a pretreatment is usually required to break up the ligno-cellulosic structure in order increase the accessibility to the carbohydrates therein contained. Many hydrothermal pretreatments have been developed so far, which treat the feedstock in the presence of liquid water or steam, all occurring in a pressurized environment. Pressure is usually attained by means of water or steam injection. Steam explosion is a widely used pretreatment, wherein steam pressure is rapidly release to cause the explosive disruption of the ligno-cellulosic structure.
Therefore, the feedstock, received dry in the conversion plant, must be transferred from atmospheric pressure to the process pressure. As the conversion process in industrial is typically conducted in a continuous way, a feedstock stream must enter a pressurized zone, while preventing catastrophic pressure losses which could perturb pressure conditions, stopping the plant operation, and cause serious injuries to operators. The pressurized zone may be the pressurized pretreatment reactor or a pressurized zone upstream of the pretreatment reactor. While different solutions have been implemented to transfer continuously the feedstock to a pressurized reactor on a pilot scale, on industrial scale stable continuous transfer is still a serious issue, as the feedstock must be transferred at very high mass flows, with typical of many tens, or hundreds, of metric tons per hours.
Another problem arises in transferring comminuted low density biomasses, such as straw feedstock, on industrial scale. Due to the low density of the feedstock, transferring a high mass flow means to handle an astonishing volume flow, in terms of cubic meters per hour. The problem becomes critical in the case that a volumetric device, such as a biomass compressor, is used to transfer the light feedstock to the pressurized section of the plant. In these conditions, the volumetric device represents a bottleneck in the transferring rate.
Different strategies have been developed so far for transferring a feedstock stream between zones at increasing pressures.
According to a first strategy, the continuous feedstock stream is partitioned in aliquots, each aliquot being sequentially transferred to the high pressure zone maintaining the two zones isolated.
As an example, U.S. Pat. No. 7,600,960 discloses a method based on sluice system, according to which the product is first conveyed through a portioning device, which produces a sequence of uniform product portions divided by uniform particle free spaces, and subsequently the product portions are conveyed individually through a sluice device. The device comprises at least one sluice chamber and two pressure locks of which at least one at any time secures a pressure tight barrier between the two pressure zones, and the product portions are force loaded from the first zone into a sluice chamber by means of a piston screw, the axis of which is practically in line with the axis of the sluice chamber, and the product portions are force unloaded from the sluice chamber and into the second pressure zone by means of said piston screw or a piston or by means of gas, steam or liquid supplied at a pressure higher than that of the second pressure zone.
The method of U.S. Pat. No. 7,600,960 is intrinsically complex, requiring a long sequence of operations to transfer a small portion of the feedstock, thereby limiting the feedstock transfer rate. Moreover, the pressure locks are subjected to failure on long operating time, and may be obstructed especially when the transfer is operated at high rate.
According to a second strategy for transferring a feedstock stream to a high pressure zone, the feedstock is first mixed with water or other liquid to form a diluted slurry, which is more easily conveyed through the different steps of the pre-treatment process under pressure. Thereby, the feedstock is transferred from the low pressure zone to the high pressure zone in a slurry form. The addition of water facilitates the transportation and mechanical handling of the lignocellulosic feedstock in unit operations upstream of and within the pretreatment reactor.
The slurry of lignocellulosic feedstock may be pressurized above the pressure in the high pressure zone using a series of specially engineered pumps.
In some cases, the diluted feedstock slurry is pumped by means of a slurry pump to an intermediate pressure and fed to a high pressure feeding device, which may be a rotary transfer device or a plug forming device, for being transferred to the high pressure zone. Examples of these solutions, specifically for a wood chips feedstock used in the pulp and paper industry, are disclosed in U.S. Pat. Nos. 5,476,572, 5,622,598 and 5,635,025 and 5,766,418. As described in these patents, using a slurry pump to feed a slurry to a high-pressure transfer device dramatically reduces the complexity and physical size of the system needed, and increases the ease of operability and maintainability.
U.S. Pat. No. 6,325,890 disclosed a system and method for feeding comminuted cellulosic fibrous material such as wood chips to the top of a treatment vessel such as a continuous digester. The disclosed method and system provide enhanced simplicity, operability, and maintainability by eliminating the high pressure transfer device used in U.S. Pat. Nos. 5,476,572, 5,622,598 and 5,635,025 and 5,766,418. Instead of a high pressure transfer device the steamed and slurried chips are pressurized using one or more slurry pumps located at least thirty feet below the top of the treatment vessel and for pressurizing the slurry to a pressure of at least about 10 bar gauge.
U.S. Pat. No. 6,325,890 thereby disclosed a method to transfer a slurry of comminuted cellulosic fibrous material directly to a digester by using a high-pressure slurry pump.
The mentioned solutions for transferring a feedstock in slurry form suffers of many drawbacks. First, the feedstock slurry is diluted and the mass of liquid present is usually at least 5 to 25 times the mass of feedstock solids present for the slurry to flow uniformly. This implies that an impressive amount of liquids should be managed on industrial scale, which can easily reach the flow rate of thousand metric tons per hour. As this may not be a serious problem at atmospheric pressure, it becomes unreasonable to pressurize and pump such flow rate by means of high pressure slurry pumps. Second, the high pressure zone is typically pressurized by steam injection at high temperature to heat up the feedstock. The amount of steam needed for this heat-up is a direct function of the total mass of the slurry, including the water addition for transportation of the slurry. Thus, the presence of a large amount of water requires a large amount of steam for the heat-up.
According to a third strategy for transferring a feedstock stream to a high pressure zone, the feedstock is transferred to the high pressure zone by means of plug forming device connected to, such as a screw press, a screw feeder, a compressor, an extruder and similar device. The feedstock is treated to reach a suitable moisture content and then inserted into the plug forming device, wherein it is advanced and continuously compressed to form a feedstock plug at outlet of the plug forming device connected to the high pressure zone. At the same time, liquids are removed from the feedstock due to mechanical actions in the plug forming device. The feedstock plug is in principle able to dynamically seal the high pressure zone preventing steam losses, but it fails to work at high flow rate on industrial scale due to frequent plug inhomogeneity. The main drawback of this solution is the fact that the sealing plug usually fails to work continuously, and plug losses are extremely frequent. Moreover, the plug loss frequency is increased at high flow rate. Another problem is represented by the high energy consumption of such device for compressing the feedstock, which also implies to dissipate the heat generated. A further problem is the efficient removal of liquids from the plug forming device, for preventing the accumulation of incompressible fluids.
U.S. Pat. No. 8,691,050 discloses methods and devices for continuously transferring particulate material into pressurized steam reactors by “flow feeder”. Material such as lignocellulosic biomass feedstocks are compacted into a “low density” plug, <700 kg/m3, which provides a dynamic seal against pressurized steam through exploitation of a steam condensation zone. The rate at which the steam condensation zone moves into the “low density” plug is offset by the rate at which compacted material is fed into the pressurized reactor. Preferred devices compact material within a flow feeder chamber by use of a loading device that works against counter-pressure provided by an unloading device. Compacted material is actively disintegrated and fed into the reactor by the unloading device. In preferred embodiments, compacted material is fed in a steady-state operation in which the interface between the steam condensation zone and the low pressure inlet zone remains stationary within the flow feeder chamber. Even if the solution disclosed in U.S. Pat. No. 8,691,050 reduces the energy required to form the plug, it fails to work in real applications, as a perfect steady state is impossible to be maintained over a long time and plug losses are increased.
U.S. Pat. No. 8,328,947 discloses a method for hydrolyzing polysaccharides in a lignocellulosic feedstock to produce monosaccharides or pretreating a lignocellulosic feedstock, in which an aqueous slurry of the lignocellulosic feedstock is fed into a pressurized dewatering zone wherein the feedstock is partially dewatered and then is compressed into a plug. The plug provides a pressure seal between the outlet of the dewatering zone and the reaction zone.
In U.S. Pat. No. 8,328,947 a method is provided for hydrolyzing polysaccharides in a lignocellulosic feedstock to produce monosaccharides or pretreating a lignocellulosic feedstock, in which an aqueous slurry of the lignocellulosic feedstock is fed into a pressurized dewatering zone by means of a high pressure slurry pump, wherein the feedstock is partially dewatered. The dewatering zone includes one or more devices to remove water under pressure from the aqueous feedstock slurry. Dewatering devices suitable for use in the invention include pressurized screw presses, as described in more detail hereinafter, and pressurized filters. The partially dewatered lignocellulosic feedstock stream from the outlet zone of the dewatering zone is moved to the inlet zone of a plug formation zone. In such zone, the partially dewatered lignocellulosic feedstock stream forms a plug that functions as a continuous pressure seal between the outlet zone of the dewatering zone and the inlet zone of the reaction zone. The plug is introduced into a reaction zone that operates at a pressure (Pr) equal to greater than about 90 psia and under suitable temperature and pH conditions to hydrolyze the polysaccharides or pretreat the feedstock. The plug provides a pressure seal between the outlet of the dewatering zone and the reaction zone. The pressure of the aqueous slurry of the lignocellulosic feedstock at the inlet of the dewatering device is related to Pr by different mathematical formulas indicated in the patent, all meaning that the pressure of the aqueous slurry is increased by the high pressure slurry pump from a starting pressure to an intermediate pressure which is closer to Pr, thereby reducing the pressure difference that the feedstock plug must continuously withstand.
The method disclosed in U.S. Pat. No. 8,328,947 presents the already mentioned drawback of pumping a diluted slurry at high flow rate with a high pressure slurry pump. Moreover, removing such at high flow rate from the pressurized dewatering zone may become an important issue.
In pretreating a lignocellulosic feedstock on industrial scale, there exist many problems related to the transferring of the feedstock in slurry form which are not solved in the prior art.
One first problem to be solved is to prevent the feedstock to plug into pressurized transferring conduits, such as pressurized pipes, particularly in the case of comminuted straw.
Another problem is the processing of a large amount of slurried ligno-cellulosic feedstock, which implies dewatering and removing a large amount of liquid in a short time. Namely, on an industrial scale, many tens, or hundreds, of tons of biomass are processed per hour and there is the need to prevent the accumulation of the removed liquid from the slurry in the dewatering devices, as the accumulation of incompressible fluids will render dewatering ineffective.
One solution to this problem is to use parallel transferring systems and devices as described in U.S. Pat. No. 8,328,947, but this simple solution increases costs. Moreover it would introduce complexity, as it would imply to manage multiple interfaces between zones at different pressure, thereby increasing the risk of frequent failure in pressurization of the high pressure zone.
Another problem to be solved is the energy needed to dewatering the slurry on industrial scale. On one hand, there is the need to reduce energy consumption for cost reasons, on the other hand if a great power is dissipated in the dewatering device, temperature would rise significantly increasing the risk of machinery failures.
A further problem is related to the transferring of a lignocellulosic slurry wherein the feedstock is a straw, such as wheat straw. Straw feedstock has typically a very low moisture content; when slurried, it adsorbs a large amount of water before free liquid appears. This adsorbed water or liquid, which is a capillary liquid is then difficult to be removed, as it is required for forming a feedstock plug capable of a sealing process zones at different pressure.
None of the above described problems arising in pretreating a ligno-cellulosic feedstock by a pretreatment process involving the transferring a ligno-cellulosic feedstock between zones at different pressures, particularly a straw feedstock, are solved by the prior art, particularly in the case of a high flow rate as required by an industrial application.